1. Field of the Invention
The present invention relates to a light-transmitting metal electrode. In detail, the invention relates to a light-transmitting metal electrode having a hyperfine structure. The present invention also relates to a process for production of the light-transmitting metal electrode.
2. Background Art
Light-transmitting metal electrodes, which have light transparency particularly in the visible region and at the same time which function as electrodes, are widely used in electronics industries. For example, all the displays distributed currently in markets, except displays of cathode ray tube (CRT) type, need light-transmitting metal electrodes since they adopt electric driving systems. According as flat panel displays typically such as liquid crystal displays and plasma displays have been explosively getting popular in recent years, the demand for transparent metal electrodes has been rapidly increasing.
In early studies of electrodes that transmit light, the electrodes were mainly made of a metal such as Au, Ag, Pt, Cu, Rh, Pd or Cr in the form of such very thin foil having a thickness of 3 to 15 nm that the metal foil could have light transparency to a certain degree. When used, for example, the thin metal foil was inserted between transparent dielectric layers for improving durability. However, since the foil was made of a metal, there was a trade-off relationship between resistivity and light-transmittance and hence it could not have properties satisfying enough to put various devices into practical use. The mainstream study, therefore, shifted to oxide semiconductors. In present, almost all the practical light-transmitting metal electrodes are made of oxide semiconductor materials. For example, indium tin oxide (hereinafter, referred to as “ITO”), which is indium oxide containing tin as a dopant, is generally used.
However, as described below in detail, the trade-off relationship between resistivity and light-transmittance is essentially still present even in oxide semiconductor materials. The problem in metal foil is that the light-transmittance decreases in accordance with increase of the foil thickness, while the problem in oxide semiconductor materials is that the light-transmittance decreases in accordance with increase of the carrier density. Accordingly, the problem to study is only changed from the former to the latter.
As described above, the demand for light-transmitting metal electrodes is expected to keep expanding in the future in many applications, but there are some future problems.
First, there is a fear that indium, which is employed as a material for the electrodes, will be exhausted. Indium is a major component of ITO, which is widely used in the light-transmitting metal electrodes, and is hence expected to be exhausted in the worldwide range according as the demand for displays typically such as thin displays increases rapidly. It is a real fact that there is a shortage of rare metals such as indium, and accordingly the cost of materials has really risen remarkably. Thus, this is a serious problem.
To cope with this problem, for example, in the sputtering process for forming an ITO film, it is studied to reuse even an ITO membrane deposited on the inner wall of vacuum chamber so as to improve the efficiency of ITO target to the utmost limit. However, techniques like that only postpone the exhaustion of indium and they by no means essentially solve the problem. In consideration of that, indium-free transparent electrodes are currently being developed. However, at present, any substitute such as zinc oxide material or tin oxide material is not yet capable of exhibiting properties exceeding ITO.
The second problem is that, if the carrier density is increased to improve electric conductively of oxide semiconductor material, the reflection in a longer wavelength region is increased to lower the transmittance. The reason for this is as follows.
According to electronic states, substances are generally classified into two types: some substances have energy gaps, and the others do not. Even when the substances having energy gaps are irradiated with light having energy smaller than the gaps, they do not absorb the light because electrons do not undergo the band transition. Therefore, with respect to visible light in the wavelength region of 380 nm to 780 nm, the substances having energy gaps of more than approx. 3.3 eV are transparent to the light.
On the other hand, depending on the width of the energy gap between the valence band and the conduction band, substances are generally categorized into three types, namely, conductors, semiconductors and insulators. The substances having relatively small band gaps are conductors, and in contrast those having relatively large band gaps are insulators, and those having middle band gaps are semiconductors. Oxide semiconductors, which are assigned to semiconductors, have chemical bonds of strong ionic character and hence generally have large energy gaps. Accordingly, they can readily satisfy the above condition at a shorter wavelength in the visible region, but the transparency at a longer wavelength is liable to lower. Further, in the case where the oxide semiconductors are used in light-transmitting electrodes, carriers of electron drift, namely, carriers of electric current are doped to obtain conductivity and transparency to visible light. For example, ITO consists of In2O3 containing SnO2 as a dopant. In this way, oxide semiconductors can be made to have low resistivities by increasing the carrier densities. However, according as the carrier density is increased, the electrode layer of oxide semiconductor as a whole becomes exhibiting metallic behavior and consequently the transmittance becomes decreasing from at a longer wavelength. Because of this phenomenon, there is a lower limit to the resistivity of light-transmitting electrodes made of oxide semiconductor.
In order to ensure transparency in the visible region, the oxide semiconductor must have a plasma frequency corresponding to a wavelength in the infrared region. This means that there is an upper limit to the carrier density. Consequently, ITO produced generally has a carrier density of n=approx. 0.1×1022 [cm−3], which is a few percent of the carrier densities of metals. The lower limit of the resistivity calculated from that value is approx. 100 μΩ·cm, and it is difficult in principle to further reduce the resistivity.
Meanwhile, it is proposed (in JP-A 1999-72607 (KOKAI)) that regularly arranged openings having a radius smaller than the wavelength of incident light be provided on the surface of highly electrically conductive thin metal foil, whereby the metal foil is made transparent to light.
Because of the aforementioned circumstances, it is desired to provide a light-transmitting metal electrode made of an electrically conductive material which is versatile and inexpensive, which is free from the fear of exhaustion and also which can keep a low resistivity, namely, a high electric conductivity.